Chromatic dispersion causes frequency-dependent group delay variations that limit data rates and corrupt transmission of optical signals within optical communication systems. As data rates of optical communication systems increase through techniques such as dense wavelength division multiplexing, chromatic dispersion measurements of optical components within the systems become increasingly important indicators of the systems' performance.
Optical components, such as optical fibers, have dispersive characteristics that vary slowly as a function of optical frequency. These components can be measured accurately using the modulation phase shift method described in Chapter 12 of "Fiber Optic Test and Measurement", edited by Dennis Derickson, Prentice Hall PTR, 1998, ISBN 0-13-534330-5. However, other types of optical components, such as fiber Bragg gratings, have dispersive characteristics that vary quickly as a function of optical frequency. Accurately measuring these components is difficult due to an inherent compromise between frequency resolution and timing resolution associated with the modulation phase shift method.
The modulation phase shift method phase compares an amplitude modulated optical carrier with a phase reference signal. The phase comparison, performed at various optical carrier frequencies, indicates frequency-dependent group delay variations within an optical component and provides the basis for chromatic dispersion measurements. However, frequency resolution is limited in the modulation phase shift method because the measured group delay is a function of the delays at the frequencies of upper and lower modulation sidebands resulting from the amplitude modulation of the optical carrier. Because the modulation sidebands are necessarily spaced far apart in frequency to achieve sufficiently high timing resolution, frequency resolution is sacrificed. Group delay variations that are near or within the frequency spacing of the modulation sidebands are smoothed out and the dispersive characteristics of the optical component are masked by the modulation phase shift method.
Frequency resolution can be increased by decreasing the modulation frequency of the modulated optical carrier. While this reduces the smoothing effect on the measured group delay, decreasing the modulation frequency decreases the timing resolution of the group delay measurement, which decreases the accuracy of the chromatic dispersion measurement. Accordingly, there is a need for a measurement scheme that achieves high frequency resolution without correspondingly decreasing timing resolution, so that the dispersive characteristics of various types of optical components can be accurately characterized.